The present invention relates to an electron guns, as for a cathode ray tube, and, in particular, to an electron gun with a resistor and a capacitor.
Performance of a cathode ray tube (CRT) depends upon the properties of the electron gun that is the source of electron beams therein, including aberrations within the electrostatic beam shaping and focusing lenses therein. Because such aberrations are related to the relatively high electrostatic potentials applied to the various grids of the electron gun, and in particular to the providing of a xe2x80x9csmoothxe2x80x9d potential gradient in the region between the focus grid and the anode. Conventionally, intermediate grids are provided between the focus grid and the anode and are biased at intermediate potentials to those of the focus grid and anode. Because these potentials are generally too high to be applied to the electron gun through pins penetrating the tube neck wherein the electron gun resides, another method is required.
Some conventional high-performance CRTs employ a high-voltage resistor connected between the anode and ground potential and tapped at a suitable point to provide a suitable bias potential for the grid intermediate the focus grid and anode. Typically, such resistors are formed of a ruthenium-oxide ink on an alumina ceramic substrate that is coated with a glaze to prevent arcing and damage therefrom. Very high resistance resistors are necessary to connect between anode potential and ground potential to drop the anode potential to an intermediate grid potential without excessive power dissipation. Typically, a resistance of about 109 ohms is suited to drop the 25-30 kV anode potential while dissipating less than about one watt. Unfortunately, the close spacing of the grids, in particular the dynamic focus grid and intermediate grid of such high-performance CRT, produces a not insubstantial parasitic capacitance therebetween, typically a few picofarads, e.g., about 2-3 pF. The dynamic focus grid is not only biased at a relatively high dc bias potential, but is also modulated by an ac voltage of several hundred volts, e.g., xcx9c500 volts, at the horizontal line scanning frequency, typically in the range of 30-100 kHz. As a result, that ac drive signal is undesirably coupled to the intermediate grid because the impedance of the parasitic capacitor is only about 106 ohms at the horizontal scanning frequency, i.e. is relatively low as compared to the resistance of the about 109 ohm resistor.
The result of this undesired coupling of the ac modulation signal also modulating the intermediate grid, and of loading from parasitic capacitance between other grids that prevents the dynamic focus grid from fully following the dynamic voltage, is that the dynamic focus grid ac modulation voltage signal must be increased substantially, by as much as 50%, to compensate for the loading of the resistively biased intermediate grid.
Accordingly, there is a need for an electron gun having a biasing arrangement that avoids or substantially reduces the undesirable effects of the parasitic capacitance between the dynamic focus grid and the intermediate grid.
To this end, the electron gun of the present invention comprises at least one cathode producing a beam of electrons, and a plurality of grids adapted to be biased at respective potentials for focusing the beam of electrons. The plurality of grids includes an anode grid adapted to be biased at an anode potential, a dynamic focus grid adapted to receive an ac signal, and an intermediate grid positioned intermediate the anode grid and the focus grid, wherein the focus grid and the intermediate grid are proximate and exhibit a value of parasitic capacitance. A resistance is coupled to the anode grid and to the intermediate grid for applying a portion of the anode potential thereto, and a capacitance coupled to the intermediate grid having a value greater than the value of parasitic capacitance.
According to another aspect of the invention, an electron lens as for an electron gun that produces a beam of electrons passing through the electron lens, comprises a plurality of electrodes through which the electron beam passes, at least one of the electrodes being a focus electrode and at least one other of the electrodes being a dynamic focus electrode. A source of a dynamic focusing signal is coupled to the one other of the electrodes for applying dynamic focusing signal thereto and a further electrode is proximate the dynamic focus electrode. A resistance having a first end adapted to be coupled to a source of bias potential and a second end adapted to be connected to a point of reference potential includes a tap intermediate the first and second ends thereof, and the tap being connected to said further electrode. A capacitance has a first electrode coupled to the further electrode and a second electrode adapted to be coupled to the point of reference potential.